The aviation industry relies upon numerous navigation aids in order safely to take off, navigate enroute, and land aircraft. Among these, the instrument landing system (ILS) is the internationally accepted and standardized navigation aid for landing aircraft at properly equipped airports. However, to allow safer aircraft take offs and landings, to provide for precision approaches at airports not equipped with ILS, and to provide more accurate navigation information, GPS is increasingly being accepted as an alternative to traditional navigation aids.
As is well known in the aviation industry, GPS is a space based radio positioning network for providing users equipped with suitable receivers highly accurate position, velocity, and time (PVT) information. Developed by the United States Department of Defense (DOD), the space-based portion of GPS comprises a constellation of GPS satellites in non-geosynchronous 12 hour orbits around the earth.
FIG. 1 shows the constellation 100 of GPS satellites 101 in orbit. The GPS satellites 101 are located in six orbital planes 102 with four of the GPS satellites 101 in each plane, plus a number of on orbit spare satellites (not shown) for redundancy. The orbital planes 102 of the GPS satellites 101 have an inclination of 55 degrees relative to the equator and an altitude of approximately 20,200 km (10,900 miles) and typically complete an orbit in about 12 hours. This positions each of the GPS satellites 101 in such a manner that a minimum of five of the GPS satellites 101 are normally observable (above the horizon) by a user anywhere on earth at any given time.
The orbiting GPS satellites 101 each broadcast spread spectrum microwave signals encoded with positioning data. The signals are broadcast on two frequencies, L1 at 1575.42 MHz and L2 at 1227.60 MHz, with the positioning data modulated using bi-phase shift keying techniques. A user receives the signals with a GPS receiver. The GPS receiver is adapted to demodulate the positioning data contained in the signals. Using the positioning data, the GPS receiver is able to determine the distance between the GPS receiver and a corresponding transmitting GPS satellite. By receiving signals from several of the GPS satellites 101 and determining their corresponding range, the GPS receiver is able to determine its position and velocity with a greater accuracy than conventional radio navaids.
The world wide availability and accuracy of GPS make the system particularly well suited for use in global flight operations and flight operations into and out of remote, unprepared or other such air strips or airports. The accuracy specifications of GPS are generally sufficient for all phases of flight down to and including non-precision approaches. Because of these characteristics, GPS receivers are increasingly being embraced by the United States defense establishment (e.g., US Air Force, Navy, etc.) and civilian carriers which require accurate, reliable navigation information in remote or unprepared areas. GPS, however, is not sufficiently accurate for vertical guidance in the precision approach and landing phases of flight.
There are several prior art methods currently in use or being considered for improving the accuracy of GPS for precision approach and landing. One such method involves using differential GPS (DGPS) techniques to improve the accuracy of the GPS signal. DGPS involves placing a local area augmentation system DGPS transmitter near the airport. The transmitter broadcasts DGPS corrections and integrity data to nearby aircraft which use the data to determine their accurate DPGS positions. Although the DGPS positions tend to be sufficiently accurate in the horizontal dimension, they have much less margin in the vertical dimension. This is primarily due to the geometric considerations of GPS. Although several satellites may be in view during a given instant, there is rarely a satellite in position to provide accurate vertical range measurements (e.g., a GPS satellite 101 directly overhead). Consequently, DGPS alone does not yield sufficient accuracy for the most demanding precision approaches (e.g., CAT-II or CAT-III precision approaches and landings).
Another prior art method involves the use of carrier phase DGPS techniques. One such prior art technique is the Integrity Beacon Landing System (IBLS) technique. Using IBLS, low power pseudolite transmitters are located on either side of the precision approach flight path to a runway. The pseudolite transmitters broadcast a signal, along with a signal being broadcast from a conventional DGPS transmitter located near the runway, such that an aircraft flying along the precision approach flight path is capable of tracking enough signal sources (GPS satellites and pseudolites) to unambiguously determine a carrier phase DGPS position accurate to within centimeters. This position is sufficiently accurate (including in the vertical dimension) to enable CAT-III precision approaches and landings. The problem with this solution, however, is that it requires expensive construction and maintenance of pseudolite transmitters, which often must occur off of the airport property.
The disadvantages of DGPS and IBLS are more significant with respect to the above stated applications (e.g., global flight operations and flight operations into and out of remote, unprepared air strips). Both DGPS and IBLS require the installation and maintenance of expensive, sensitive electronic ground-based transmitters in the airport vicinity. This equipment must be properly maintained in order to be relied upon. Both DGPS and IBLS require dependence upon these locally installed and maintained ground-based transmitters. In certain situations, particularly with regard to military applications, such dependence is unacceptable.
Additionally both DGPS and IBLS are completely nonfunctional in the event GPS signals are lost, jammed, interfered with, or otherwise unavailable. Both systems require the presence of receivable GPS signals in order to provide augmented positioning accuracy. Further, neither DGPS or IBLS adds any redundancy to the navigation systems already present on board a typical user aircraft (e.g., civilian air carrier or military transport). DGPS and IBLS typically augment an existing GPS receiver during the approach and landing phases as opposed to providing a separate, additional source of navigation information for added redundancy.
Thus, what is required is a system which provides improved positioning accuracy with respect to GPS. What is required is a system which provides sufficient accuracy in the vertical dimension to allow precision approaches and landings. The required system should be independent of any ground-based transmitters. In addition, the required system should provide an added degree of redundancy, providing an additional source of navigation information in conjunction with other pre-existing aircraft navigation instruments (e.g., inertial, ILS, TACAIN, etc.). The present invention provides a novel solution to the above requirements.